Heat treating process for a 2% fe 2% cr 2% mo titanium base alloy



United States Patent O HEAT TREATING PROCESS FOR A 2% Fe 2% Cr 2% Mia-TITANIUM VBASE ALLOY Donald John'West and Richard Augustus Baughman,

Cincinnati, Ohio, assignors to General Electric Company, a corporation of New York'- Application May25, 1955, Serial No. 511,115 y 4 Claims. (Cl.l14 813) This invention relates to a nrocess kfor heat Vtreating titanium and in particular to a process for heat treating a particulary type of titaniumalloy in order to make it ductile.-

Compressorwheels of a turbojet engine are a critical part vof theengine and are subjected to high migratory stresses when rotating at high velocity. Also, large stress concentrations take place when the turbojet engine undergoes sudden bursts of thrust during transient operation. The points of stress concentrations take place in areas where there is a tendency for the stresses to build up. If the material undergoing these stress concentrations has, ductility, the material can undergorplastic deformation v.at the point of stress concentration and relieve the stress. If the stress concentration continues to buildup and is not relieved, it can result in the form of an initial` fracture which will later. destroy .the part and perhapsdestroy the engine..4 This problem becomes particularly critical during the high speeds of a jet engine at .which time the temperature and velocity ofvrotating parts is also exceedingly high, so that. any initial fracture. will b e vseriously aggravated and undoubtedly4 do a lot of damage to the engine. Since titanium vallow .has a good strength weight ratio, the use of compressorlwheels made from titanium alloys is very desirable in order .to

achieve considerable Weight savings in the engine. t How-l ever, one of the undesirable features of titanium is that it tends to remain brittle lmainly because of they impuritiescontained in the alloy itself. Therefore, the 'ductility necessary to meet the specification requirements isa problem confronting the entire titanium industry.v

It is therefore a general object of this invention to provide a process for, heat treating titanium articles to give them the necessary ductility required to comply with'.

the specifications. v

In forging titanium, the articleV previously has been formed high in thebeta and alpha plus beta range to give the material the necessary ductility.V However, in

formingarticles in the beta and alpha plus; beta range, it has vbeen difficult, to consistently obtain theproperties Therefore large amounts of titanium articles eral, approximately 5.0% of the compressor wheelsmade of titanium and heat treated to the previous processes` fail tomeet the specification requirement `for ductility andare rejected. of this invention to provide a process for heat treating titanium articles to consistently meet the required duca tility.

It is therefore another general object A further object of this invention isy to provide, a

tionof fine alpha structure v in order to obtain;improv ed-,

ductility.

It is another object of this invention to providefa heat. treating l process for'anvvalloyrin-,which-the materialy is heated to .a temperature in vthebeta field to form a beta;A structure .and then transformed isothermally at. a teme... peratureinthealphavplus beta'phase to producev a ductile: material.

These and other objectswill become more apparent. when read .in the vlight of the accompanying specification taken -in connection .with l the attachedA drawing. The .I scope of our finventionwill be pointedout in `the appended c1aims.

The .drawing is a time-temperature-transformation diagram for vthe titanium alloy which includes Aabout .2% iron, 2% chromium -and2% molybdenum. v

In general, the vinvention may be practiced by heating, an alloy comprising about 2% iron, about 2% chro. mium, about ,2% molybdenum ,the balance essentially titanium toa temperature exceeding the alpha plus .beta transformation temperature, maintaining thealloy. (at.` this temperature for a period of time, and then. transformingthe -alloy isothermallyto an alphaplus beta structure at a lower temperature.

In order to heat treat titanium alloy articles made of approximately 2% of` iron, 2% of chromium, v2% of molybdenum vand the rest titanium and its impurities,V- so as to have the necessary ductility to meet -the required,- specifications, the first step is betatizing thev article by heating to 1650 F. into the-area marked beta` in the; drawing. Betatizing is a term used in connection with heat treating a material in the beta field. Itis noted. that 4 theoxygen caninuence the transformation tem-,l peratureas well as the contaminations found. in the material. These factors could raise vthe beta temperature. It is desirable vhowever to heat the material in the beta iield while staying as low in the beta field as-possible. The primary reason for doing this. is to form a beta structure with a minimum amount of alpha phase before transforming back intoA the alpha plus beta eld as shown in the drawing. To get rid of all of the alpha, it is necessary to stay at this low temperature inthe beta field, otherwiseiine alpha may rema inand. will resultin accompanying low ductility of the material. 'There fore, by staying at the low temperature in the beta field, the tine alpha will be eliminated to a substantial extent so that the material in general contains beta before transforming back to an alpha plus beta structure at the lower temperature. In betatizing a material it is noted that the time in general is not too critical so long as the material is maintained at this temperature sufciently long to be stabilized at that temperature.

The next step in the process is the isothermal heat treating portion in which `the'materialis cooleddownfV to 1325? F. Thiscan be done by transferring to another furnacewhich-is already at the-1v325 F. temperature or staying in thefurnace and'cooling-it down to 13259 F. Either method of cooling hasbeenfound satisfactoryso long as the material is not permitted to cool down below 1325 F. At 1325 F. coarse alpha, shown by the shaded portion in the drawing, is allowed to form. It is noted that the 1325 F. temperature is very critical in this'instance in that dropping below the 1325 F. temperature, line alpha will for m It is this fine alpha that forms in the; structural composition of this material when in the alpha plus beta phase that Vcauses brittleness'v 1 when cooling down to room-temperature. Also, -it is noted that if the material is held` ata highertemperaturm. than 1325 F., less alpha forms in reaching equilibrium conditions, and more of it stays beta so that upon sub# sequent cooling the additional beta transforms to line alphaand causes,.brit`tleness. The timev at 1325 F. vfor' the: heskresultsfis foundito lhe x13 rhours.. This'Y time'gtis `l' critical and it is necessary to maintain an isothermal .is approximately 11 hours, depending on the material itself so long as equilibrium is reached and coarse alpha is formed. Y e

' After stabilizing the material at 1325 F. the material is` then furnace cooled to 1200 F. This step is necessary since after-cooling from `1325"k F. additional alpha may be formed. In other words, at any lower temperature, more alpha is present at equilibrium conditions than at 1200 F. or 1325 F. The beta to alpha transformation is a `time-temperature phenomenon. `In order to have alpha and beta in equilibrium at a given temperature, sucient time for transformation must be allowed. Also, the lower the temperature, the slower the reaction. Long times at 1200 F. are therefore necessary to allow the alpha and beta phases to reach equilibrium in the alpha plus beta region shown in the drawing and prevent the formation of an unstable beta phase upon subsequent cooling to room temperature. If insufcient time is allowed at 1200F. and unstable beta forms, then a flue precipitate or line alpha will form from the unstable beta during subsequent engine operation, causingV loss of ductility.

It is therefore necessary to cool the material to 1200 F. and stabilize this material by transforming it into the alpha plus beta phase at that temperature for 24 hours. Although the material can cool down to room temperature from the 1325 F. and then be heated to 1200 F., it has beenfound that cooling to 1200 F. instead of dropping down to room temperature, tends to keep scaling of the titanium down to a minimum.

The last step of the process is cooling the material 'to room temperature by taking the article out of the furnace. It is noted that cooling down to room temperature by taking the article out of the furnace is much better than furnace cooling since brittleness again results when furnace cooling is employed thereby making the ductility of the material low.

The following table which is the summary of tests conducted on six different heats of material compares the physical properties of the titanium base alloy containing about 2% iron, about 2% chromium and about 2% molybdenum before and after `our novel heat treatment. Themarlced increase in ductility is especially to be noted:

1 Before After Tensile Strength (p. s. i.) 139, 600 132,100 0.2% Yield Strength (p. s. i.) 13 1, 600 123, 200 0.02% Yield Strength (p. s. 1.). 119, 100 113, 300 Percent Elongation 12, 21.0 Percent Reduction in Area 17.0 31.0

In order to show the stability of this particular Ytitanium alloy after our novel heat treatment process, a series of compressor wheel rings were aged for up to about 1000 Additional test results' showing the increase in ductility resulting from the use of our novel heat'treatment is 4 shown by the following table which summarizes a series of tests performed on compressor discs:

Before Heat Treatment After Heat Treatment Sample Series Percent Percent Percent Percent Elongation Reduction Elongation Reduction In Area In Area l\/I 13) 11 r14 .21 30 R 8 12 15 15 L 130 5 7 15 15 In summary therefore, a step treatment for heat treating titanium is provided to result in aV titanium alloy of improved ductility in which the material is heated to its lowest point in the beta range and held at this temperature until the material becomes all beta. The next step is furnace cooling to 1325 F. and holding the temperature until as much coarse alphathat is going to form.

at that temperature takes place. The material is then furnace cooled again to 1200 F. andheld for a sufficient length of time to produce the alpha plus beta structure. This stabilizes the material and prevents tine alpha from being precipitated. The elimination of fine alpha prevents brittleness and eventual age hardening during engine lSi The last step is cooling the material in air operation. to room temperature.

It is noted that the heat treatment temperatures of 1325 F. and 1200 F. must be maintained for suicient periods of time to produce ductilityV and provide a stable alpha plus beta microstructure.

The above example of the invention'has been given by way of illustration and is not intended as a limitation of the invention. The time and temperatures given can vary within narrow limits according to the characteristics of the material, such variations being intended to be in cluded within the spirit and scope of this invention. Y

What we claim as new and desire to secure by Letters Patent 0f the United States is:

l. The process of heat treating a titanium alloy including about 2% iron, about 2% chromium and about 2% molybdenum comprising heating to approximately 1650*PA F. to form the beta phase, maintaining this temperacluding about 2% iron, about 2% chromium and about 2% molybdenum, heating to the lowest temperature in the beta range and holding at this temperature until all of the material becomes beta, cooling to about 1325 F. and holding this temperature for approximately 13 hours to permit as much alpha as is going to form to take place, furnace cooling to 1200 F. to stabilize the maten'al to prevent age embrittling, air cooling this material to room temperature.

3. In a process for heating titanium alloy consisting of- 2% iron, 2% chromium, 2% molybdenum and the rest titanium and its impurities, heating the material to the beta range to form as much beta as possible, cooling the material to the alpha plus beta range without permitting tine alpha to form, annealing at 1200 F. and maintaining this temperature long enough to `stabilize the material to prevent age embrittling, and air cooling the material to room temperature.

4. The process of heat treating a titanium alloy includl' ing about 2% iron, about 2% chromium and about 2% molybdenum comprising heating' at approximatelyy 1650 F. for about one hour, furnace cooling to about 1325 F., maintaining 1325 F. for approximately 13 hours to permit coarse alpha phase to form in equilibrium with beta phase without permitting ne alpha to form, air cooling to room temperature, heating to 1200 F. and maintaining this temperature for about 24 hours to prevent flne alpha from forming and then air cooling to room temperature.

References Cited in the file of this patent UNITED STATES PATENTS Jafee et al. May 13, 1952 Vordahl July 10, 1956 OTHER REFERENCES 

1. THE PROCESS OF TREATING A TITANIUM ALLOY INCLUDING ABOUT 2% IRON, ABOUT 2% CHROMIUM AND ABOUT 2% MOLYBDENUM COMPRISING HEATING TO APPROXIMATELY 1650* F. TO FORM THE BATE PHASE, MAINTAINING THIS TEMPERATURE UNTIL SUCH TIME AS THE MATERIAL BECOMES STABILIZED, FURNACE COOLING TO 1325* F. AND MAINTAINING THIS TEMPERATURE FOR APPROXIMATELY 1O HOURS PERMITTING COARSE ALPHA PHASE IN EQUILIBRIUM WITH BETA PHASE TO FORM WITHOUT PERMITTING FINE ALPHA TO FORM, COOLING TO 1200* F. AND MAINTAINING THIS TEMPERATURE FOR 24 HOURSS TO PREVENT FINE ALPHA FROM FORMING, AND AIR COOLING TO ROOM TEMPERATURE. 